The present invention generally relates to an x-ray imaging system. In particular, the present invention relates to a system and method for x-ray imaging with spatial modulation of the x-ray beam.
Conventional x-ray imaging systems consist of an x-ray source exposing an object to an essentially uniform x-ray beam. As the beam passes through the object, varying radiographic densities throughout the object cause varying portions of x-ray flux to be attenuated (for example, absorbed or scattered) in the object. After passing through the object, the remaining beam strikes a detector. As the detector receives the beam with varying intensities, the detector measures and communicates the beam intensities to a data acquisition system. The data acquisition system may then use the beam intensities to create a shadow image.
Several fundamental problems exist in this conventional approach. For example, the entirety of the imaged object receives a relatively high x-ray dose independently of varying radiographic thicknesses throughout the object, regardless of the presence of motion in imaged objects and/or the degree to which various object volumes are of interest to the viewer.
A large dose is commonly used to ensure that the object volumes that attenuate the largest amount of the beam receive sufficient photon flux to provide an image of those volumes. If a beam striking an object volume with a large radiographic thickness has insufficient intensity to allow a sufficient number of x-ray photons to reach the detector, then the resultant shadow image may not produce sufficient contrast for features in the object volume. A sufficient number of photons must reach the detector to allow differentiating objects' radiographic thickness variations from fluctuations in the detected numbers of photons. These fluctuations are known as quantum noise or mottle.
However, the high x-ray doses also strike object volumes with smaller radiographic thicknesses, which require much less dose to be imaged adequately. Excessive exposures of the thin object volumes may be harmful. In addition they may cause additional imaging problems, such as, for example, (a) increased x-ray scatter, (b) increased veiling glare, and (c) detector saturation. Current high-performance x-ray detectors may allow imaging object volumes with both large and small radiographic thicknesses without saturation. However, such systems may still expose object volumes with smaller radiographic densities to unnecessarily large x-ray doses. In addition, such high-performance detectors add considerable expense to an x-ray system.
Another problem with conventional x-ray imaging are high doses to object volumes imaged for reference only without the need for high spatial and grayscale resolution. These volumes may be imaged with a decreased dose rate and still provide adequate information while object volumes that require high grayscale and spatial resolutions may still need to be exposed to usual doses.
Another problem with conventional fluoroscopy is excessive exposure rates to object volumes where little change occurs from frame to frame and, therefore, little new information is present. If an image region is known to contain little object motion, it may be possible to reduce dose and increase information reuse from previous frames to render an accurate representation of the object. Moving or changing object volumes may still need to be exposed to regular dose rates to provide adequate image quality.
Several beam modulation techniques have already been proposed. These techniques may be classified into two general categories based on the goals they pursue: (a) Beam Equalization methods attempt to equalize or homogenize the detector exposure spatially; and (b) Region-of-Interest Radiography and Fluoroscopy methods attempt to reduce exposure to anatomical volumes of lesser clinical interest. Some examples of each will be given below.
Another categorization of beam modulation methods is based on whether or not the displayed image is compensated for the introduced brightness modulation. In many applications this compensation is unnecessary as the uncompensated images are of equal or greater value to the user as the uncompensated images. In other applications, it may be necessary to present image intensities that accurately represent true radiographic thicknesses in the imaged objects and, before presenting the output image, the system may need to reverse the intensity variation introduced into the x-ray beam.
Beam modulation methods may also be categorized based on whether the beam modulation is configured and invoked automatically or manually. Thus, automatic and manual beam modulation methods are distinguished.
Several techniques have been proposed to equalize or make uniform the exposure to the x-ray detector for the purpose of dose reduction, x-ray scatter reduction, or to prevent detector saturation. These techniques typically consist of placing an equalizing beam filter between the x-ray source and imaged objects. For example, in Sirvin, U.S. Pat. No. 5,185,775, entitled “X-ray Apparatus Including a Homogenizing Filter”, a filter matching the morphology of the imaged object is placed between the x-ray source and the imaged object to homogenize detector exposure and to improve the quality of angiographic images.
Several technologies have been proposed to quickly produce filters matching the morphology of arbitrary objects. One such technology is disclosed in Boone, U.S. Pat. No. 5,107,529, entitled “Radiographic Equalization Apparatus and Method.” Boone describes the utilization of a plurality of juxtaposed discs used in the filtration of an x-ray beam. Each disc includes a complex attenuation pattern and is individually rotatable in order to obtain numerous attenuation patterns. Based on a single scout image, discs are rotated so as to create an optimal attenuation pattern. The attenuation pattern provides for increased beam attenuation in areas of the imaged object corresponding to overexposed areas of the preliminary image. In this way, Boone describes an x-ray filtering apparatus and method for equalizing x-ray beam intensity received at a detector.
Another proposed solution is disclosed in Edholm et al., U.S. Pat. No. 3,755,672, entitled “Exposure Compensating Device for Radiographic Apparatus.” Edholm describes an x-ray filter that may alter an amount of x-ray absorption. The filter has a variable shape such that the amount of x-ray absorption within different portions of the filter can be independently altered. In addition, the amounts of x-ray absorption in portions of the filter are automatically adjusted in response to signals based on a preliminary or scout image detected by radiation detecting means located below the imaging plane. Edholm therefore describes an x-ray filter that can automatically alter an amount of x-ray attenuation based on x-ray intensities detected during a preliminary image.
Another proposed solution is disclosed in Dobbins, III, U.S. Pat. Nos. 4,868,857 and 5,081,659, entitled “Variable Compensation Method and Apparatus for Radiological Images.” Dobbins describes the modulation of an x-ray beam based on a preliminary or scout low-dose x-ray image. As above with regards to Boone and Edholm, Dobbins therefore describes a static x-ray filtration method and apparatus. The modulation is based on a digital beam attenuator mask that provides for an x-ray beam that is equalized when received at the detector. The digital beam attenuated mask of Dobbins is combined digitally with detected x-ray intensities to form a final x-ray image.
Region-of-Interest Fluoroscopy (“ROIF”) has been proposed to address the problem of excessive exposures to less important object volumes (e.g. Rudin et al, “Region of Interest Fluoroscopy”, J. of Med. Phys., 1992 September-October; 19(5):pp. 1183-9). In ROIF, a procedure-specific filter is placed between the x-ray source and the imaged object to selectively attenuate the x-ray beam in regions of lesser clinical interest. Prior to the procedure, compensating mask images are acquired by taking an image of the attenuating filter alone. During the procedure, the mask image is subtracted digitally, similarly to digital subtraction angiography techniques, to recover true attenuations of the imaged object.
Many of the proposed systems require human intervention to produce or select beam filters, to position them in the beam, and to perform image compensation. Several solutions have been proposed to automate portions or the entirety of the beam equalization process. These solutions collectively are known as Computed Equalization Radiography. Some categories of such solutions are: (a) scanning or raster systems (e.g. Vlasbloem et al, “AMBER: A Scanning Multiple-Beam Equalization System for Chest Radiography”, Radiology, vol. 169, No. 1, pp. 29-34), (b) solutions using x-ray absorbing liquids or deformable substances whose volumetric shapes are controlled mechanically or electronically (e.g. Tang, Mather and Zhou, “Area x-ray beam equalization for digital angiography”, J. of Med. Phys., 1999, 26(12):pp. 2684-92), (c), printing desired attenuation patterns with x-ray absorbing ink, (Hasegawa et al., “Geometrical properties of a digital beam attenuator system”, Med. Phys. 14: 3, 314-21, May-June, 1987) (d) solutions that use multi-leaf or multi-layer semitransparent filters of varying thickness whose positions are adjusted independently to produce desired attenuation patterns (e.g. Boone, U.S. Pat. No. 5,107,529, entitled “Radiographic Equalization Apparatus and Method”).
The above references describe beam modulation techniques, in which the required x-ray intensity field is computed from a preliminary scout image or is programmed manually. However, as many x-ray procedures may require hundreds or thousands of continuous frames from multiple views, these solutions do not provide a mechanism for uninterruptible point-and-shoot imaging with optimized beam modulation.
Some of the proposed solutions such as raster-beam or slit-beam scanning systems (such as AMBER) significantly increase x-ray tube loading requirements because only a small portion of the x-ray beam is used at any time.
Solutions that use semitransparent substances to selectively attenuate the beam are sensitive to the photon energies in the x-ray beam. Filters designed to attenuate the x-ray beam with effective x-ray photon energies around 35 keV would be too opaque for meaningful beam modulation when the effective photon energy is dropped to, for example, 20 keV, or too transparent when the effective photon energy is increased to, for example, 70 keV. Addressing the problem with specialized filters that work with low- and high-energy beams would require a substantial increase in the complexity of such systems. The amounts or thicknesses of these x-ray absorbing substances would need to vary by significant factors when the x-ray technique undergoes a significant change. For such systems to provide meaningful beam modulating factors in a wide range of x-ray techniques, their designs may be prohibitively complex.
In addition, automated beam modulation systems proposed in above references may be too bulky, slow, and expensive to provide high speed, resolution, and dynamic range that would make them useful in a wide spectrum of imaging applications.
To make a beam modulation system useful in dynamic imaging environments such as medical interventional imaging, a need exists for an improved system and method allowing for modulation of an x-ray beam continuously without user intervention and without the need for a scout shot. Such a system and method can control the x-ray beam intensities across the field of view prior to the x-ray beam striking the imaged object. The degree of variation may need to be sufficiently high, for example, up to one or two orders of magnitude while resolving a sufficient number of intermediate intensity values in a wide range of x-ray techniques. The system and method may also automatically reduce the x-ray exposure to regions of an imaged object where a lower dose is sufficient to adequately render features of interest, such as in radiographically thin, static, or less interesting regions, for example. The system may also render the displayed image without compromising various aspects of image quality, distracting the viewer, or distorting displayed images. In short, such system can deliver the benefits of beam equalization and region-of-interest fluoroscopy (for example, reduced dose, reduced x-ray scatter, reduced optical glare, and reduced saturation) while making the displayed images appear as if produced with a uniform high-exposure beam. In addition, such a system and method can provide for improved image quality by irradiating with higher doses object volumes of interest, object volumes with high radiographic thickness, and object volumes with anticipated motion.